Description of problem:
It is possible to create two instances with same language within a
single bundle, by clicking the buttons 'fast'.

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Create an english item
2. Go to languages tab
3. Click 'change  language'
4. Before request in 3. has completed, click 'add language'
  
Actual results:
There are now two items in bundle with German language

Expected results:
Error display that there is already a german instance

Additional info:
Here is the SQL invariant that is violated

select count(*) as result 
  from (
    select count(*) as count, parent_id, language
      from cms_items i, cms_bundles b
     where i.parent_id = b.bundle_id
       and version in ('live', 'draft')
     group by parent_id, language
  ) sub where count != 1